﻿I. INTRODUCTION. 



The experimental work which constitutes the basis of this report was 

 undertaken with the object of throwing further light upon certain prac- 

 tical problems in relation to pest immunity, and particularly Math the 

 idea of determining the most efficacious method of protective inoculation 

 against plague. The subject of immunization against pest is not only 

 of general scientific interest, but at least to several tropical and sub- 

 tropical countries is of great practical importance. One need only recall 

 the mortality in India of nearly a million deaths from this disease during 

 the past 3 r ear (1905) to be impressed with the importance of the problem, 

 and while it is true that at present no epidemic of plague exists in the 

 Philippine Islands, yet sporadic cases occur from time to time and it 

 must be recalled that only a few years ago (1903) it was thought by 

 the Board of Health advisable and necessary to perform among the 

 Chinese of the city of Manila general inoculations against this disease. 



It is true that the subject of protective inoculation against plague has 

 received considerable attention during the past few years and that prophy- 

 lactics have been recommended by several authors, but, while it is 

 admitted that by their use a certain degree of pest immunity can be 

 produced and demonstrated in a number of the more insusceptible animals 

 and, occasionally, even in those very susceptible to this infection, never- 

 theless, it has sometimes seemed questionable whether we were able, by 

 the inocultion of these prophylactics, to obtain in man an immunity of 

 such a degree as to be protective against the- natural and usual methods 

 of infection by the malady. Thus, Kolle called attention to the fact 

 that since, by the use of even very large doses of the killed pest organisms 

 only exceptionaly were we able to immunize against pest infection the 

 animal which is most susceptible to plague, namely, the guinea pig, it 

 seemed very doubtful if favorable results could be obtained in man, where 

 relatively much smaller doses of the killed cultures were injected. Yet, 

 from 1898, when protective inoculation was first introduced, up to the time 

 of my first report on vaccination against plague in November, 1905, no 

 other methods except those in which the killed cultures were employed 

 had been used in the very extensive human inoculations performed against 

 this disease in India and in other countries. 



In the latter part of the year 1903, when the Board of Health of 

 Manila was practicing among the Chinese in this city protective inocula- 

 tion against plague by the injection of the killed cultures of the pest 

 bacillus, the method at that time carried on in Japan and consisting of 



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